Products made from or incorporating plastic are a part of almost any work place or home environment. Generally, the plastics that are used to create these products are formed from virgin plastic materials. That is, the plastics are produced from petroleum and are not made from existing plastic materials. Once the products have outlived their useful lives, they are generally sent to waste disposal or a recycling plant.
Recycling plastic has a variety of benefits over creating virgin plastic from petroleum. Generally, less energy is required to manufacture an article from recycled plastic materials derived from post-consumer and post-industrial waste materials and plastic scrap (collectively referred to in this specification as “waste plastic material”), than from the comparable virgin plastic. Recycling plastic materials obviates the need for disposing of the plastic materials or product. Further, less of the earth's limited resources, such as petroleum and polymers, are used to form virgin plastic materials.
When plastic materials are sent to be recycled, the feed streams rich in plastics may be separated into multiple product and byproduct streams. Generally, the recycling processes can be applied to a variety of plastics-rich streams derived from post-industrial and post-consumer sources. These streams may include, for example, plastics from office automation equipment (printers, computers, copiers, etc.), white goods (refrigerators, washing machines, etc.), consumer electronics (televisions, video cassette recorders, stereos, etc.), automotive shredder residue, packaging waste, household waste, building waste and industrial molding and extrusion scrap.
Different types of plastic parts are often processed into shredded plastic-rich streams. The variety of parts can vary from a single type of part from a single manufacturer up to multiple families of part types. Many variations exist, depending on at least the nature of the shredding operation. Plastics from more than one source of durable goods may be including in the mix of materials fed to a plastics recycling plant. This means that a very broad range of plastics may be included in the feed mixture. Some of the prevalent polymer types in the waste plastic materials are acrylonitrile-butadiene-styrene (ABS), high impact polystyrene (HIPS), polypropylene (PP), and polycarbonate (PC), but other polymers may also be present.
Some additives can be included in the recycled plastic materials based on the color of the waste plastic material from which the material is derived, and/or the conditions (e.g., heat, light, chemicals) to which the waste plastic material was intended to be exposed. Often, titanium dioxide along with low levels of antioxidants and possibly optical brighteners are present in plastics. The titanium dioxide is used to make the plastic white, optical brighteners may be used to hide any yellowing, and antioxidants are used to prevent degradation during processing and over the life of the part. Mixtures of recycled plastics tend to have white plastics containing titanium dioxide pigments and black plastics containing carbon black. In addition to white and black materials, waste plastic streams can include materials in a variety of other colors, such as blue, green, brown, orange, yellow and red, which contain a variety of pigments and dyes that give them their color.
Some additives found in recycled plastics mixtures are chemical compounds of heavy metals such as cadmium (Cd), lead (Pb), mercury (Hg) or chromium (Cr). Many of these heavy metal-containing additives are no longer added to virgin plastics due to health concerns and regulations restricting their use, but they can be present at significant levels in recycled plastics mixtures.
Regulatory limits exist for the concentrations of these elements applications including electronics, packaging and toys. The RoHS directive (2002/95/EC), for example, limits the concentration of Cd to 100 ppm and the individual concentrations of Pb, Hg and Cr (VI) to 1000 ppm in electronics sold in Europe. The European Union Directive 94/62/EC on Packaging and Packaging Waste limits the sum of the concentrations of Cd, Pb, Hg and Cr (VI) to 100 ppm.
At the same time that these strict limits on the content of heavy metals are coming into effect, market and legislative forces are encouraging manufacturers to incorporate post-consumer plastics into their products.
In order to satisfy these requirements, it is important to identify and implement appropriate methods to reduce the content of heavy metals in plastics recovered from mixtures of post-consumer durable goods.
In the following, methods are described for the selective reduction of the content of Cd and other heavy metals in mixtures of plastic flakes.